This invention relates to a system and method for monitoring machine tool operations to detect different classes of tool breaks that may and may not require interruption of the cutting process.
The phenomenon of cutting tool breakage is a major obstacle to the automation of machining processes. In most machining operations that have no automatic tool break detection system, it is necessary to have an operator for each machine tool to promptly detect took break events and take action to prevent them from causing serious damage to the workpiece or the machine tool, or from putting a computerized numerical control (CNC) machine into a situation resulting in repeated tool breaks caused by excessive depth of cut at the point where the break occurred. Automation of the machining operation, with lower levels of human surveillance of individual machine tools, requires the automation of the tool break and machine response functions. A Machine Tool Monitoring (MTM) system has been developed for the automation of these functions and is covered by many commonly assigned patents and copending applications. It senses high frequency vibrations generated by the cutting process, and applies signal processing and pattern recognition logic techniques to detect major tool break events that make an immediate significant change in cutting conditions that is reflected in a major shift in the mean vibration level. In order to avoid false alarm problems from normal cutting artifacts in the vibration signature, it deliberately ignores other tool break events that do not make an immediate significant change in cutting conditions.
The MTM system was originally developed for applications using hard but brittle ceramic tools in aggressive cutting of tough aerospace materials, and therefore it assumed that each cut would be made with a new cutting edge. Under these circumstances it is not only acceptable, but actually preferable, to have the automatic tool break detection system ignore tool breaks that do not make an immediate significant change in cutting conditions. However, in most machining applications, tool cutting edges are used for several successive cuts before a tool change is made. This change in operating conditions changes the desired functionality of the automatic tool break detection system. Now, when a tool breaks and does not immediately change cutting conditions very much, it is usually desirable to replace the tool at least before the start of the next scheduled cut.
Two different types of situations of this type have been encountered. Ceramic tools, including fibre-reinforced ceramic tools, occasionally suffer delamination breaks that do not have much effect on the cutting process. However, they weaken the tool and make it more likely to break on the next cut. Both ceramic and carbide tools sometimes suffer major fractures, but are held together by cutting forces for some time after the fracture, with little effect on cutting conditions. When the cutting forces are relieved at the end of the cut, or when they undergo changes at the start of the next cut, the different pieces of the fractured tool separate and one or more of them may fall away. Thus, cutting conditions may be radically changed by the tool break, but the change may not take place until long after the initial tool fracture event. The MTM tool break detection system tends to ignore the initial tool fracture in these cases, and generally will not alarm until cutting conditions change. If this change does not take place until the start of the next scheduled cut, MTM may have some difficulty detecting it at all because there is no normal cutting period on that cut, and MTM basically looks for changes from the assumed normal pre-break cutting conditions to the assumed abnormal post-break conditions.
For most automated machining operations, in which tools are used for several successive cuts between tool changes, an automated tool break detection system is needed with the capability of detecting not only tool breaks that result in an immediate major change in cutting conditions, but also tool breaks that do not do so, but should nonetheless cause the tool to be replaced before the next cut is started. The MTM system is very flexible, and it can be adjusted to detect many of the tool breaks that do not cause an immediate major change in cutting conditions. Only a few such tool breaks, where the tool fracture transient is obscured by normal cutting noise, cannot be detected. The major problem with this solution is that, in order to set the MTM system to alarm on tool fracture transients not followed by vibration signal indications of a major change in cutting conditions, it is necessary to significantly increase the probability of false alarms from normal cutting vibration signal artifacts that mimic the tool fracture vibration signatures.
Systems that use information in a vibration signal and in another signal sensing another physical quantity for purposes of tool break and wear detection have been proposed. Copending allowed application Ser. No. 153,300, filed Feb. 5, 1988, C. E. Thomas, D. G. Wildes, and M. Lee, "Cutting Tool Wear Detection Apparatus and Method", utilizes a spindle power or force signal to detect tool wear and a specific type of tool break, namely crumble breaks of ceramic tools. It also makes use of both high frequency and low frequency vibration signals for the same purposes. It does not utilize fast transients in any of these signals and is not designed to detect abrupt tool fracture events. British patent No. 2,140.951A discloses that three machining parameters, ultrasonic emission by the cutting process, main drive power, and feed drive power, provide for a clear differentiation between process irregularities and tool breakage. A simultaneous abrupt variation of the measured values of all three parameters indicates a tool break, and a concurrent variation of two of the three parameters for a predetermined time indicates a process irregularity.